Orthometric height is the vertical distance measured from the geoid, an equipotential surface of Earth’s gravity field that approximates mean sea level, to a point on Earth’s surface, following the curved path of the local plumb line. This physically meaningful height definition corresponds to the intuitive concept of elevation and determines the direction water flows, making orthometric height essential for engineering, construction, hydrological, and topographic applications.

The distinction between orthometric height and ellipsoidal height (the native output of GNSS receivers) is fundamental to proper use of satellite positioning for elevation-dependent applications. GNSS determines position relative to a mathematical ellipsoid, producing ellipsoidal heights that have no direct physical significance regarding gravity or fluid flow. The geoid surface, by contrast, is an equipotential surface where gravity is everywhere perpendicular, water at rest naturally conforms to this surface, and water flows from higher to lower orthometric heights.

Converting GNSS-derived ellipsoidal heights to orthometric heights requires knowledge of the geoid undulation (N), the separation between ellipsoid and geoid surfaces, at each location. The relationship is: orthometric height (H) = ellipsoidal height (h) – geoid undulation (N). Geoid undulation varies globally by approximately ±100 meters, and locally by meters to tens of meters within a single project area. Accurate conversion requires applying an appropriate geoid model such as EGM2020 (global) or GEOID18 (United States).

For professional applications, proper handling of orthometric versus ellipsoidal heights prevents serious errors. Construction projects must establish drainage grades based on orthometric heights to ensure water flows in intended directions. Flood mapping references mean sea level (geoid-related), not the ellipsoid. Topographic maps show orthometric elevations. Surveying projects connecting to existing benchmark networks must work in orthometric heights or apply careful transformations. Understanding the conceptual and mathematical relationships between these height systems is essential knowledge for any GNSS professional working with elevation data.